1. Technical Field
The present invention relates to a dot impact printer that prints on a recording medium, and relates more particularly to a dot impact printer that can reduce variation in the platen gap even when the printhead unit is repeatedly installed and removed.
2. Related Art
Printers used in photo minilabs that have image printers for printing photographs include correction value printers (CVP) and back printers for printing specific information on the back side of paper (recording medium) on the front side of which an image is printed by an image printer.
Dot impact printers that print by driving the head pins of the recording head against the recording medium with an ink ribbon therebetween are commonly used as back printers in such minilabs. Dot impact printers are simple and small, and therefore suited to printing specific information such as the date printed or an image ID number.
Dot impact printers used as back printers are normally disposed well inside the printing system, which uses the image printer as the main printing device. To facilitate replacing the ribbon cassette and other maintenance operations, the printhead unit on which the wire dot head and ribbon cassette are carried can be positioned to the printing position and the ribbon replacement position using a linkage mechanism or other type of unit conveyance mechanism.
When using a wire dot recording head (wire dot head) that prints by striking the paper with head pins (wires) through an ink ribbon, however, the size of the gap between the platen and the wire dot head (the platen gap) affects print quality, particularly the print density and multipart form printing capacity. Precisely adjusting the platen gap in such printers is therefore necessary.
Japanese Unexamined Patent Appl. Pub. JP-A-H10-202981 therefore teaches inserting a gap gauge between the platen and the distal ends of the wires of the wire dot head to optimally adjust the gap therebetween in such dot impact printers.
Dot impact printers conventionally used as back printers are, however, disposed deep inside the printing system, and the printhead unit must be removed using a unit conveyance mechanism every time the ribbon cassette is replaced. The platen gap between the wire dot head carried on the printhead unit and the platen disposed to the paper conveyance path inside the printing system is therefore not stable, possibly resulting in such problems as variations in the print density and a drop in wire doll head durability.